Nucleic acid amplification reactions have emerged as powerful tools in a variety of genetic analyses and diagnostic applications. The value of these techniques is their ability to rapidly increase the concentration of target nucleic acids of interest that might be present at very low and otherwise undetectable levels. For instance, by utilizing the polymerase chain reaction (PCR) amplification technique, one can amplify a single molecule of a target nucleic acid by 106 to 109.
PCR is perhaps the most well-known of a number of different amplification techniques. This well established procedure involves the repetition of heating (denaturation) and cooling (annealing) cycles in the presence of a target nucleic acid, primers that hybridize to the target, deoxynucleotides, a polymerase and cofactors such as metal ions. Each cycle produces a doubling of the amount of the target DNA. The cycles are conducted at characteristic temperatures: 95° C. for denaturing double stranded nucleic acid, 50 to 65° C. for hybridization of primer to the target nucleic acid, and 72 to 77° C. for primer extension (see, generally, Sambrook et al., (1989) Molecular Cloning: A Laboratory Manual, 2nd ed., Cold Spring Harbor Laboratory Press; see also U.S. Pat. Nos. 4,683,202 and 4,683,195, for example).
Methods for conducting PCR amplifications fall into two general classes. The approach typically utilized is a time domain approach in which the amplification reaction mixture is kept stationary and the temperature is cycled (see, e.g., Cheng, et al. (1996) Nucleic Acids Res 24:380-385; Shoffer, et al. (1996) Nucleic Acids Res. 24:375-379; and Hong, et al. (2001) Electrophoresis 22:328-333). While methods utilizing this approach can be conducted with relatively small sample volumes, the methods require complex regulation of heater elements and relatively long reaction times. Another approach that has been discussed is limited to a space domain approach in which three temperature zones are constantly kept at the different temperatures and the reaction mixture runs in a serpentine flow channel above it (see, e.g., Kopp et al. (1998) Science 280:1046-1048). A method such as this can be conducted at relatively high speed because it is not necessary to heat and cool the heaters, but requires the use of relatively large sample volumes.